The present invention relates to a novel organopolysiloxane not known in the prior art and not described in any literatures, a method for the preparation thereof and a curable composition containing the same as the principal component. More particularly, the invention relates to an organopolysiloxane composition capable of giving a cured rubbery elastomer having outstandingly high mechanical strengths and an organopolysiloxane as the principal ingredient of the composition as well as the method for the preparation of the organopolysiloxane.
As is well known curable organopolysiloxane compositions are widely used in a variety of applications in the fields of electric and electronic industries, automobiles and other machines, building constructions and others by virtue of the excellent heat and cold resistance, mold-releasability, electric insulation and other respects possessed by the cured products thereof. A problem in these curable organopolysiloxane compositions is the relatively low mechanical strengths of the cured products thereof so as to greatly limit the applicability of the compositions.
In this regard, compounding of a reinforcing filler such as a finely divided silica flour with the organopolysiloxane is effective though being accompanied by some disadvantages in other respects of increase in the consistency, deplastification and loss of tranparency. Several attempts have been proposed to improve the mechanical properties of cured organopolysiloxane compositions by modifying the chemical structure of the organopolysiloxane as the principal ingredient of the composition. For example, a proposal has been made for a so-called addition-type curable organopolysiloxane composition, in which crosslinks are formed between a diorganopolysiloxane having silicon-bonded vinyl groups and an organohydrogenpolysiloxane having hydrogen atoms directly bonded to the silicon atoms by the addition reaction therebetween, that the vinyl groups in the former polysiloxane are bonded to the diorganopolysiloxane molecules in an uneven distribution to be localized in particular positions. Such a method is not free from some disadvantages that the diorganopolysiloxane having vinyl groups in a localized distribution is expensive due to the difficulties in the preparation thereof since it is synthesized by the deamination reaction of a vinyl-containing diorganopolysiloxane terminated at both molecular chain ends each with a silanolic hydroxy group and a non-vinyl diorganopolysiloxane with an aminocontaining organosilicon compound having, in a molecule, two amino groups of the formula SiNR.sub.2, R being a monovalent hydrocarbon group. Moreover, tne deamination reaction is hardly controllable in respect of the viscosity or degree of polymerization of the product which may be eventually gummy to give poor workability to the composition compounded therewith.
Several other methods are known for the preparation of such a diorganopolysiloxane containing vinyl groups in a localized distribution in the molecule including the method of equilibration between an organopolysiloxane of high vinyl content such as hexavinyl disiloxane and a cyclic poly(diorganosiloxane) in the presence of an alkali catalyst and a method in which a vinyl-containing organocyclotrisiloxane such as trivinyl trimethyl cyclotrisiloxane is polymerized with an organolithium compound or a diorganotrisiloxane such as a tetracyclotrisiloxane, which is a penta-coordinate silicon complex. The former method, however, is disadvantageous because the number of the vinyl groups in the resultant organopolysiloxane cannot exceed three in a molecule so that the cured product thereof is unsatisfactory in respect of the mechanical strengths. The latter method is also disadvantageous because a sufficiently large degree of polymerization of the organopolysiloxane product is obtained only after an unduly long reaction time and eventual condensation reaction and equilibration between the molecules as formed may result in a final product of which the degree of polymerization exceeds the target value. In this case, moreover, the polymerization reaction is greatly affected by the moisture so that a disadvantage is unavoidable because the process must be undertaken with an extreme care for excluding the moisture eventually entering the reaction mixture under a condition of dehydration.